Liquid crystal displays (LCDs) are optical displays used in devices such as laptop computers, hand-held calculators, digital watches and televisions. Some LCDs, for example used in as laptop computers, cell phones, and some smaller computer monitor and television screens, are illuminated from behind using a backlight that has a number of light sources positioned to the side of the display panel. The light is guided from the light sources using a light guide that is positioned behind the display. The light guide typically includes some arrangement for extracting the light from the light guide and directing the light towards the display panel. This arrangement is commonly referred to as an edge-lit display.
One important aspect of the backlight is that the light illuminating the display panel should be uniformly bright. Illuminance uniformity is particularly a problem when the light sources used at the edge of the backlight are point sources, for example LEDs. The light guide is typically designed to spread the light within the light guide so that the display has no dark areas. This problem is less acute when extended light sources are employed, for example, fluorescent tubes, although it is still necessary to ensure that the amount of light extracted per unit area is uniform across the display.